This invention relates to fuel cells and, in particular, to an externally manifolded fuel cell system adapted to impede the flow of electrolyte from the fuel cell stack of the system to the manifold used with the stack.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Fuel cells operate by passing a reactant fuel gas through the anode, while passing oxidizing gas through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
A fuel cell stack may be an internally manifolded stack or an externally manifolded stack. An internally manifolded stack typically includes gas passages for delivery of fuel and oxidant gases built into the fuel cell plates. In an externally manifolded stack fuel cell plates are left open on their ends and gas is delivered to the cells by way of manifolds sealed to the respective faces of the fuel cell stack. The manifolds in each type of fuel cell stack provide sealed passages for delivery of fuel and oxidant gases to the fuel cells and prevent those gases from leaking to the environment and to the other manifolds. These functions of the manifolds must be performed under the operating conditions of the fuel cell stack and for the duration of the stack life.
The fuel cell stack is electrically conductive and has an electrical potential gradient along its length such that one end of the stack is at a positive-most electrical potential (the positive potential end of the stack) and the other end is at a negative-most electrical potential (the negative potential end of the stack). External manifolds, which are typically made from metallic materials, must therefore be electrically isolated from the fuel cell stack so as not to short circuit the stack. Electrical isolating assemblies, which include dielectric insulators and one or more gaskets, have been used between the metallic manifold and the fuel cell stack to produce the desired electrical isolation. A typical external manifold system includes three to four manifolds each employing similar electrical isolation assemblies to provide similar seals and dielectric isolation for each of the manifolds.
A schematic exploded view of one manifold and an electrical isolating assembly in a typical arrangement for a conventional externally manifolded fuel cell system 100 is shown in FIG. 1. As shown, the system 100 includes a fuel cell stack 1, a manifold comprising a metallic manifold 6 which covers a face 1a of the stack 1 and an electrical isolating assembly 101 disposed between the stack 1 and the manifold 6. The assembly 101 includes a dielectric member 5, a wet gasket 2 abutting the stack face 1a, a ceramic block or member 3 abutting the wet gasket 2 and a dry gasket 4 disposed between the ceramic block 3 and the dielectric member 5 in an abutting relationship. The other manifolds of the fuel cell system use a similar design.
In liquid electrolyte fuel cell systems, such as for example molten carbonate fuel cells, electrical isolation provided by the electrical isolating assembly 101 may be severely compromised when liquid electrolyte in the fuel cells migrates from the stack to a point where it wets the components of the isolating assembly abutting the manifold 6. In particular, during fuel cell operation, the stack face 1a becomes wet with liquid electrolyte, which is absorbed by the wet gasket 2. The ceramic block 3 comes into contact with liquid electrolyte through its surface abutting the surface of the wet gasket 2. When the electrolyte is transported across the ceramic gasket to reach the dry gasket 4, the dielectric capacity of the ceramic block 3 is substantially reduced. As a result, electrical isolation between the manifold 6 and the stack 1 becomes difficult to maintain with the dry gasket being responsible for most of the voltage drop between the stack 1 and the manifold 6. This voltage drop may be as high as 500 Volts.
The electrolyte migration from the stack face 1a across the electrical isolating assembly 101 is facilitated by the difference in electrical potential between the fuel cell stack and the manifold. Generally, the manifold has a constant electrical potential floating between the positive-most and the negative-most electrical potentials of the stack. This causes the manifold to be at a lower potential than the positive potential end of the stack. As a result, a positive electrical potential is applied between the stack and the manifold. This, in turn, promotes the flow of electrolyte from the stack into and through the electrical isolating assembly 101.
More particularly, the electrical potential at the positive potential end of the stack 1 leads to formation of carbonate ions (CO3=) as follows:CO2+½O2+2−→CO3=
These carbonate ions are attracted to the lower electrical potential at the manifold 6, this lower electrical potential being shown as a negative electrical potential relative to the stack positive potential. An “electrochemical pump” is thereby created which facilitates the flow of the electrolyte from the positive potential end of the stack across the electrical isolating assembly 101 towards the manifold 6. This results in wetting of the block 3, the gasket 4 and the dielectric insulator 5 with electrolyte so as to compromise their dielectric properties and degrade the electrical isolation ability of the assembly 101.
Conventional electrical isolating assemblies have been adapted to counteract this electrolyte flow by utilizing, for example, a smooth ceramic block for the block 3, as is disclosed in U.S. Pat. No. 6,413,665. Such a construction has prolonged the ability of electrical isolation assemblies to maintain their electrical isolation characteristics by delaying the wetting of the isolating assembly components adjacent the manifold. However, over time, electrolyte migration can still occur. Accordingly, additional ways of impeding the migration of electrolyte from the stack through such electrical isolating assemblies are still being sought.
It is therefore an object of the present invention to provide a fuel cell system adapted to further reduce the migration of electrolyte through an electrical isolating assembly proving electrical isolation between a fuel cell stack and the manifold used with the stack.
It is a further object of the present invention to provide a system of the aforementioned type in which reduction of electrolyte migration is accomplished in a simple and cost effective manner.